1. Field of the Invention
The present invention relates to communication systems, and more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems are typically configured as cellular communication systems which comprise a plurality of cells each of which contains system equipment that is part of the wireless communication system. A cell is a geographical area with defined boundaries and contains system communication equipment such as a base transceiver station (BTS) that conveys (i.e., transmits and/or receives) communication signals to and from mobile equipment. The system equipment comprises typical communication equipment such as transmitters, receivers, transceivers, and processors that convey communication signals within the communication system. The system equipment for a cell may be located at one or more points in the cell. The terms “base transceiver station” and “system equipment” may be used interchangeably. The term “system equipment” may also include non-base transceiver station entities, such as a base station controller (BSC). The system equipment is usually owned, operated and controlled by a service provider or a network provider. Examples of service providers include wireless telephone companies, data network companies and other communication service entities. The mobile equipment is usually owned, operated and to a certain extent, controlled by a user of the communication system. Examples, of mobile equipment include cellular telephones, wireless personal computers, pagers and Personal Digital Assistants (PDA). The information conveyed between the mobile equipment and the system equipment comprise communication signals representing voice, video, and data information. The data information can be textual information, graphical information or any other type of information that is not time sensitive. Voice and video signals represent information that is time sensitive; that is, voice and video signals are conveyed in a continuous manner without any time gaps. When time gaps occur during the transmission of voice or video signals, the gaps are discernible making it difficult for a user to properly interpret the information being carried by the communication signal.
As mobile equipment migrates from cell to cell within the system, the mobile equipment, at some point, may be handed off or switched (depending on the type of traffic being exchanged) from one base transceiver station to another base transceiver station. Hand off and switching procedures are well known concepts. A handoff procedure is used for voice in many different types of cellular wireless communication systems such as Code Division Multiple Access (CDMA) systems, Frequency Division Multiple Access (FDMA) systems and Time Division Multiple Access (TDMA) systems. In CDMA systems, the handoff procedure is termed soft, which implies that the mobile may be in bi-directional voice communication with a multiplicity of base transceiver stations for a period of time as it migrates from one cell to another. A mobile positioned to receive voice information from and transmit voice information to multiple base transceiver stations at the same time is said to be in handoff.
While in a particular cell, mobile equipment is served by the base transceiver station of that cell; that is, information is conveyed between the mobile equipment and the base transceiver station. Information is transmitted from the base transceiver station to the mobile over a communication link commonly referred to as the forward link. Information is received by the base transceiver station from the mobile over a communication link commonly referred to as a reverse link. The forward link—as well as the reverse link—comprises different communication channels for conveying either traffic information or signaling information. Traffic information is the information being received or transmitted by the user. As previously indicated, examples of traffic information include voice, video and data. Signaling information is information generated by the system equipment and also by the mobile equipment to allow the system to control, operate and manage the flow of traffic information in the system. The signaling information is arranged, transmitted and received in accordance with a protocol. A protocol is a set of rules that dictate how signaling information is to be formatted, transmitted and received. A protocol is usually based on a standard defined by standards bodies comprising private commercial entities and/or governmental regulatory bodies.
The handoff procedure identifies particular circumstances under which the system allows a mobile equipment to be simultaneously served by more than one base transceiver station, these base transceiver stations are known as the members of the active set of base transceiver stations. For relatively high-speed data applications, it is preferable to send data to the mobile from one of these base transceiver stations; this base transceiver station is designated the serving base transceiver station for the data application. At some point in time, the migrating mobile equipment may choose to receive data from another base transceiver station (i.e., another cell) in the active set; this base transceiver station is designated as the target base transceiver station for data service. In the context of a CDMA system complying with CDMA 2000 Release C standard, the mobile receives data on the Forward-Packet Data Channel (F-PDCH) from a serving base transceiver station. At some point during the cell switching procedure, the serving base transceiver station stops transmission on the F-PDCH to the mobile. At approximately the same time, the target base transceiver station becomes the serving base transceiver station and resumes transmissions to the mobile on the F-PDCH. The mobile is thus said to have switched from a serving base transceiver station to a target base transceiver station completing a cell switching procedure. In other words, the mobile has switched cells or the mobile has switched from one cell to another cell. The cell switching procedure is thus performed by the mobile, the serving system equipment and the target system equipment.
During the cell switch, various parameters are received by the serving base transceiver station from the mobile. Also the target base transceiver station receives parameters from the serving base transceiver station. One of the parameters received by the serving base transceiver station from the mobile is the Carrier to Interference ratio (C/I) for the serving base transceiver station. The Carrier to Interference ratio is a measure of a pilot signal that is transmitted in a substantially continuous manner by all of the base transceiver stations in the communication system. The serving base transceiver station, the target base transceiver station as well as all of the other base transceiver stations in the system transmit a pilot signal in a substantially continuous manner over their respective forward pilot channel that is part of the forward link. The mobile equipment being served by the base transceiver stations receive the pilot signals, measure the pilot signals and transmit the measurement (in the form of Carrier to Interference ratio or C/I) for the serving base transceiver station (in the cdma2000 Release C standard). C/I is a measure of the quality of the forward channels in the forward link. Generally speaking the higher the C/I, the better the quality of the forward channel. While a mobile is being served by a base transceiver station, the mobile transmits the C/I to the serving base transceiver station for a defined period of time followed by the transmission of channel measurement adjustment information for another defined period of time. The channel measurement adjustment information is information that is used by the serving base transceiver station to either increase or decrease its transmission power (i.e., the power of its transmitted signals over the forward link), bandwidth, data speed and allocation of time interval on a communication channel so as to adequately and efficiently serve the mobile.
In many CDMA systems, the mobile transmits information to a base transceiver station during a defined time period called a frame. For some of these CDMA communication systems, the frame has a duration of 20 msec. The frame is divided into 16 equal time slots. Thus, each time slot is approximately 1.25 msec. in duration. For such systems that use 20 msec. frames, the mobile transmits the C/I information during the first time slot using 4 bits. Each of the ensuing 15 time slots of the frame are used by the mobile to transmit channel measurement adjustment information that, for example, enables the serving base transceiver station to adjust the power, bandwidth, data speed, and intervals of time allocated to the mobile on the forward data channel.
During the cell switch procedure, the migrating mobile equipment transmits the C/I of the serving base transceiver station. A decreasing C/I measurement is typically an indication that the mobile is moving away from the serving base transceiver station and that the serving base transceiver station should significantly increase the power of its communication signals in order to adequately serve the mobile. The mobile is also tracking the C/I of a target base transceiver station. Usually, when the C/I of the target base transceiver station is increasing, it is an indication that such target base transceiver station is a good candidate to adequately serve the migrating mobile. In CDMA systems that mainly convey data, the mobile can wait until the C/I of the target base transceiver station is sufficiently adequate to initiate and complete the cell switch, taking advantage of the dynamics of the system and decreasing the likelihood of loss of communication and/or an increased occurrence of errors while in handoff. One particular CDMA communication system that conveys mainly data information is the cdma2000 Release C standard previously under development as cdma2000 EVolution for Data and Voice.
Generally, in CDMA communication systems, and in particular, in cdma2000 Release C CDMA communication systems, information transmitted and received by each mobile is coded with a code called a PN (Pseudo-Noise) code which in effect defines a particularly specific communication channel for each mobile in the system. Further, when a mobile wishes to transmit information to a specific base transceiver station, it codes the information with a spreading code called a cover code that specifically identifies that base transceiver station. The spreading code is a code which when applied to a communication signal causes the frequency spectrum of the signal to encompass a relatively wider bandwidth than the bandwidth of the signal without the applied spreading code. One type of spreading code is the well known Walsh code used in CDMA systems. Thus, information transmitted by a mobile to a particular base transceiver station is coded with a PN code identifying the mobile and with a cover code identifying the base transceiver station for which the information is destined. The information is carried by one or more sinusoidal signals (e.g., radio signals) which are modulated and combined in accordance with the value of the PN code and the cover code to form a specific waveform defined by those codes.
Normally, when the mobile does not wish to switch cells, it transmits C/I information of the serving base transceiver station to that base transceiver station during the first time slot of the frame using the cover code of the serving base transceiver station. The C/I information is a 4-bit quantized value of the measured C/I information of the serving base transceiver station. There are currently 8 defined cover codes 6 of which are used by CDMA communication systems that comply with the cdma2000 Release C standard. The remaining two cover codes are not, at the present, used by cdma2000 Release C communication systems. The mobile then transmits 1-bit update information (i.e., channel measurement adjustment information) in each of the ensuing 15 slots of the frame communicating the change in channel quality to the serving base transceiver station. During this time of normal transmission, the frames being transmitted by the mobile are called non-switch frames. For cdma2000 Release C CDMA systems, the frame is contained in a radio signal (combined and modulated by the cover code and the PN code) transmitted by the mobile over a signaling channel of the reverse link called the Reverse Channel Quality Indicator (R-CQICH).
When a migrating mobile wishes to initiate cell switching it notifies the serving base transceiver station in some manner. One way in which the migrating mobile notifies the serving base transceiver station is to transmit a fixed and known value (during, say, slots 14 and 15 of the 20 msec. frame) to the serving base transceiver station using the cover code of the target base transceiver station. Each of the remaining time slots (i.e., 13 remaining time slots) of the frame are used, as before, by the mobile to transmit channel measurement adjustment information about the serving base transceiver station. The C/I information is represented by a 4-bit block of data that is a quantized value of the measured C/I value of the forward link of the serving base transceiver station. The transmission can be received by both the serving and target base transceiver stations. The format of the frames as discussed above (i.e.,two slots transmitted with the cover code of the target with a fixed value describes what is called a switch frame. Therefore, during normal operation (i.e., cell switch is not desired by mobile or network equipment), non-switch frames are transmitted by the mobile over the R-CQICH signaling channel. However, when a mobile desires to switch cells, it transmits switch frames having the format described above. A number of changes may be made to the format of switch frames without substantially altering the mechanism for detecting switching indication by the mobile at the network. For example, the location and number of switch indication slots within a 20 ms frame may be changed based on a directive from the base transceiver station. Also, the transmitted bits during these slots may be either held fixed or represent C/I adjustment information for the serving base transceiver station, or even represent C/I information for the target base transceiver station.
As described above, at any given time, a cdma2000 Release C mobile receives data on the forward packet data channel (F-PDCH) from a single BTS sector. This sector is termed the serving or source BTS sector. At any time during the data call, the mobile may designate a different BTS sector in its active set, from which it desires to receive the F-PDCH. The BTS sector, so designated, is termed the target BTS sector. This action by the mobile is known as cell re-selection or cell switching. A cdma2000 Release C mobile cell signals cell switching via specially formatted transmissions, on the Reverse Channel Quality Indicator Channel (R-CQICH). The current mechanism is shown in the following figure.
Referring to FIG. 1, there is shown the format of non-switch frames (CQICH frames) and switch frames (CQICH/switch indication frame) transmitted by a mobile in a cell A that is switching to cell B. The first slot of the non-switch frame indicates the 4-bit quantized C/I information for the serving cell (i.e., cell A is contained in such slot).
In particular, the first slot includes the Walsh spreading function for sector A for a full C/I update for sector A and the gain for a full C/I update. The ensuing 15 slots of non-switch frame 1 contain, inter alia, 1-bit channel measurement adjustment information for the serving cell A. The ensuing 15 slots include the Walsh spreading function for sector A including differential C/I updates 1-15 for sector A and the gain for the differential C/I updates. In the switch frame, the first slot includes the Walsh spreading function for sector A for a full C/I update for sector A and the gain for a full C/I update. Slots 14 and 15 include the Walsh spreading function for sector B and repeat a fixed value (+1) using the gain for cell switch indication. The remaining 13 slots contain channel measurement adjustment information for serving cell A. The remaining 13 slots include the Walsh spreading function for sector A including differential C/I updates 1-13 for sector A and gain for the differential C/I update. It is noted that there is an interruption in the adjustment information when the switch frame is being transmitted. Non-switch frame 2 is transmitted by the mobile when cell switching is completed meaning that cell B is now serving the mobile. The first slot of non-switch frame 2 contains 4-bit C/I quantized information for the forward link of cell B. This slot includes the Walsh spreading function for sector B including full C/I update for sector B and the gain for the full C/I update. The remaining 15 slots contain 1-bit adjustment information for cell B. The remaining 15 slots include the Walsh spreading function for sector B including differential C/I updates 1-15 for sector B and the gain for the differential C/I update.
Note that multiple consecutive switch frames (from 1 to 16) may be transmitted by the mobile to indicate a switch. The serving base transceiver station also has the ability to release the mobile and allow it to move to the target base transceiver station before all the switch indication frames have been transmitted by the mobile.
In conventional systems, the serving BTS sector attempts to detect a switch indication by a mobile. Detection of a switch indication results in
(i) transfer of data intended for the mobile (as well as current state of the data transmission and retransmission protocol) to the target BTS from the serving BTS sector, and
(ii) subsequent transmission on the F-PDCH to the mobile from the target BTS.
Since power control of the R-CQICH obeys the or-of-the-downs rule, there is no guarantee of reliable reception at the serving BTS sector. Further, the reverse link channel feedback and acknowledgement channels in cdma2000 Release C systems, that support operation of the forward link packet data channel, do not benefit from handoff reception across non-co-located BTS sectors.